With the increasing availability of smaller and smaller sensors and microcontrollers, reactive-type systems have been integrated into vehicles which ensure better vehicle stability. Such systems include, for example, ABS (anti-lock braking system), EBD (electronic brake-force distribution) and so on.
Wheel tires have also recently been used to house sensors capable of improving vehicle safety. One example of a known system is TPMS (tire pressure monitoring system), which monitors the pressure in each tire and, if it decreases, alerts the driver through a warning lamp lighting up in the instrument panel.
A further quality jump is currently being made: in fact, said reactive systems are being gradually replaced by pro-active systems. The latter are capable of evaluating in advance the behavior of the vehicle according to an integrated model that includes the road, the vehicle, the tire and the environment.
As the complexity of the safety system increases, the number of sensors to be arranged in the tire increases as well and, in addition to problems of mechanical nature posed by the integration of said sensors, one must face the problem of supplying power to the radio node used for transmitting the collected data. Early systems were rather simple and used small tire-integrated batteries, the life of which was similar to that of the tire itself.
In the new safety systems that also involve the vehicle's tires, more energy is required because, in addition to supplying power to the data transmission radio node, it is also necessary to feed microcontrollers built in the sensors, which pre-process and filter the multitude of collected data. The life of the batteries currently available is not sufficient to meet the new power specifications.
More in general, low-cost energy generation for feeding a vehicle's electric component is nowadays a subject of intense research.
As to the state of the art of batteryless systems used for feeding devices integrated into a vehicle's wheels, in particular in the tires, the literature can be subdivided into three categories.
The first category includes the so-called “energy harvesting” systems. Such systems collect the mechanical stresses and vibrations of the rotating tire and transduce them into electric energy using suitable devices, typically of piezoelectric nature.
The systems belonging to this first category are relatively simple, but the quantity of energy extracted is small and often insufficient to meet the energy requirements of the most advanced tire-integrated electronic systems.
The second category of batteryless systems for feeding devices integrated into a vehicle's wheels includes magnetic induction systems. Such systems are based on the dynamo principle. In fact, these systems comprise a winding installed in the tire, and a magnet is arranged in the body, near said winding. The turning tire generates a voltage across the winding, which voltage is then stabilized and used for feeding the electronics integrated into the tire itself Although the systems belonging to said second category can provide a good power level, they require additional components which are typically both costly and bulky, such as, for example, ultracapacitors, used for storing energy that can be used when no energy is available, as is the case, for example, when a vehicle is temporarily standing still at a traffic light.
The third category of batteryless systems for feeding devices built in a vehicle's wheels includes radiofrequency energy transfer systems. Such systems exploit the possibility of transferring radiofrequency energy from a transmitter located in the vehicle to a receiver circuit integrated into the tire, which detects the radiofrequency field and converts it into direct current, necessary to power the tire-integrated electronic circuitry.
In this context, solutions have been presented which range from radiofrequency signals of just a few MHz, such as RFID transponders, to microwave signals of many GHz, which are converted into direct current by a special device called rectenna.
This class of solutions, while ensuring that sufficient energy can be transferred to feed tire-integrated sensors, has some drawbacks, such as the possibility of radio interference with other vehicle's onboard equipment, limited transfer efficiency due to the presence of thick metallic parts of the vehicle that shield the propagation of the electromagnetic signal, and dispersion of the irradiated energy outside the spatial sector concerned by the transfer. Some patent applications belonging to said third category are, for example, U.S. patent applications Nos. 2006/0197655, 2007/0262856, 2007/0222571 and U.S. Pat. No. 7,202,778.
More in particular, U.S. Pat. No. 7,202,778 describes a wireless system for monitoring the pressure of an aircraft's tires. Each wheel of the aircraft comprises two resonant circuits which can be interrogated by two resonant circuits arranged on the aircraft, wherein one circuit emits a frequency sweeping signal and the other one receives the response resonance peak. The two wheel-mounted resonant circuits have different resonant frequencies; therefore, when the frequency sweeping means on the aircraft side start the scanning process, they encounter two distinct resonance peaks. The scan starts at a frequency of 14 MHz; the frequency then gradually increases until the two resonance peaks of the wheel-side resonant circuits are encountered. Each resonance peak is temporarily locked by a PLL (Phase Locked Loop), which acquires its frequency value and stores it into a register. A processor calculates the frequency difference between the two resonance peaks and from this information obtains the tire pressure. However, this architecture is very complex from the electronic viewpoint, in that it requires four resonant circuits per wheel to which power must be supplied, plus a number of processing units.
Evanescent waves are also known in the art, defined as waves present in the immediate vicinity of an antenna, i.e. in the non-radiative near field. The energy of evanescent waves is emitted, and almost totally reabsorbed, in a cyclic manner. These waves are said to be evanescent because the effects of their presence decrease exponentially as the distance from the antenna increases; at a distance as short as approx. one third of their length, they are no longer detected. Evanescent waves are discussed, for example, in “Wireless power transfer via strongly coupled magnetic resonances”, Andrè Kurs, Robert Moffat, Peter Fisher, Aristeidis Karalis, J. D. Joannopoulos and Marin Solja{hacek over (c)}ić, published in the journal “Science”, Vol. 317, 6 Jul. 2007, pp. 83-86.